The present invention relates to a transformer which shields its secondary electrical noises in its primary, more especially to a noise-shielded transformer which suppresses electrical noises by absorbing conductible electrical noises that flow into the power line to cause interference. A noise-generating device becomes a noise source to other peripheral electronic devices and the electrical noises from such a device, which may become a power line disturbance, is regulated as an electromagnetic interference since the electrical noises have a bad effect on other peripheral electronic devices.
Electronic devices, which are not noise-generating sources, are regulated by electromagnetic susceptibility since such electronic devices are subject to a software malfunction or a hardware breakdown due to external electrical noises.
Accordingly, measures against electrical noises are required for reliability enhancement and lifetime protection of various electronic devices. That is, a noise-shielded transformer is required which can prevent electrical noises generated by a noise-making device from flowing into other peripheral devices in order to protect them, and also protect devices used as loads against external noises.
FIG. 1 is a plan view showing a conventional noise-shielded transformer, and FIG. 2 is a circuit diagram for the transformer of FIG. 1. As shown in FIGS. 1 and 2, the conventional noise-shielded transformer shown includes a primary winding 2 having a predetermined number of turns wound around a shield winding 3 of the same number of turns as the primary winding 2 toroidal core 1. A is wound around primary winding 2. A secondary winding 4 of the same number of turns as the primary winding 2 is wound around shield winding 3, and a second shield winding 5, wound in the same manner as the first shield winding 3, is wound around secondary winding. Both ends of the first and second shield windings 3 and 5 are connected to a ground lead terminal 6.
In FIG. 2, reference numerals 7a and 7b are lead terminals of the primary winding 2 to which an AC power source is applied, and 8a and 8b are lead terminals of the secondary winding 4 which are connected to a load.
The operation of the conventional noise-shielded transformer as above is described below with reference to the equivalent circuit diagram of FIG. 3.
When an AC power source including a pulse-property noise is applied to the primary winding 2, the pulse-property noise generates a magnetic flux of a high frequency which includes current flow in the primary winding 2. At this time, the toroidal core 1 minimizes the magnetic flux of a high frequency generated by the pulse-property noise since the toroidal core 1 is made of the material that sharply decreases the magnetic permeability over high frequencies.
And, since the internal shield winding 3 is grounded, the pulse-property noise in current flowing through the primary winding 2 is directed ground through the static capacitance C of the shield winding 3.
In the meantime, since a zero potential ground line is formed around the secondary winding 4, when an external electromagnetic field acts upon primary winding 2, noise induction to the secondary winding 4 due to the external electromagnetic field can be prevented.
Also, since the first and second shield windings 3 and 5 are divided respectively into shield windings 3a, 3b and 5a, 5b, circulating current does not flow in the interior even when a bais voltage induced, and when an inequilibrium noise signal is flows between the lead terminals 7a and 7b of the primary winding 2 and the ground an inverse electromotive force is generated so as to suppress the generation of a noise voltage.
However, such a conventional noise-shielded transformer has disadvantages in that the noise-eliminating effect therein is low since the toroidal core has a very low leakage flux and it is difficult to prevent the noise induction with only the external shield winding against the external electromagnetic field, and this noise-suppressing effect is insufficient to eliminate the capacitance between the primary and secondary windings against the noise of the common mode, and the workability is not good because the internal and external shield windings 3 and 5 are divided into two halves, respectively, and are inversely connected in series.